The U.S. Government has rights in this invention pursuant to National Institute of Health grants HL-39478, HL-39297, HV-38045, and HL-34962.
This invention relates generally to magnetic resonance imaging, and more particularly the invention relates to spin-echo imaging using hyperbolic secant pulses.
Nuclear magnetic resonance (NMR) imaging, also called magnetic resonance imaging (MRI), is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is completely non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
A descriptive series of papers on NMR appeared in the June 1980 issue of the IEEE Transactions on Nuclear Science, vol. NS-27, pp. 1220-1255. The basic concepts are described in the lead article, "Introduction to the Principles of NMR," by W. V. House, pp. 1220-1226, which employ computed tomography reconstruction concepts for reconstructing cross-sectional images A number of two-and three-dimensional imaging methods are described. Medical applications of NMR are discussed by Pykett in "NMR Imaging in Medicine," Scientific American, May 1982, pp. 78-88, and by Mansfield and Morris, NMR Imaging in Biomedicine, Academic Press, 1982.
Briefly, a strong static magnetic field is employed to line up atoms whose nuclei have an odd number of protons and/or neutrons, that is, have spin angular momentum and a magnetic dipole moment. A second RF magnetic field, applied as a single pulse transverse to the first, is then used to pump energy into these nuclei, flipping them over, for example to 90.degree. or 180.degree.. After excitation the nuclei gradually return- to with the static field and give up the energy in the form of weak but detectable free induction decay (FID). These FID signals are used by a computer to produce images
The excitation frequency, and the FID frequency, is defined by the Larmor relationship which states that the angular frequency .omega..sub.o, of the precession of the nuclei is the product of the magnetic field B.sub.o, and the so-called magnetogyric ratio, .gamma., a fundamental physical constant for each nuclear species: EQU .omega..sub.o =B.sub.o .multidot..gamma.
Accordingly, by superimposing a linear gradient field, B.sub.z =z .multidot. G.sub.z, on the static uniform field, B.sub.o, which defines Z axis, for example, nuclei in a selected X-Y plane can be excited by proper choice of the frequency spectrum of the transverse excitation field applied along the X or Y axis. Similarly, a gradient field can be applied in the X-Y plane during detection of the FID signals to spatially localize the FID signals in the plane. The angle of nuclei spin flip in response to an RF pulse excitation is proportional to the integral of the pulse over time.
Due to inhomogeneity of the static field and due to the gradient fields, nuclei spins in a selected slice can become dephased due to differences in precession. The spin echo technique has been introduced to overcome the dephasing of the spins. Briefly, after applying an initial 90-degree pulse to flip nuclei spins at a right angle to the static and gradient field, a 180-degree pulse is applied to turn the dephased spins over into a mirror-image position. The 180-degree pulse is applied after a time period, .tau., following the initial 90-degree pulse, and after a time period, 2.tau., following the initial 90-degree pulse the spins are refocused and create a "spin echo" which can be sensed for imaging purposes.
Heretofore, adiabatic (180-degree) pulses have been used to invert spins in the presence of both RF and B.sub.o inhomogeneity. Hioe and Silver et al., Physical Review 1984, independently determined that the Bloch equation
an analytic solution when driven by the complex hyperbolic secant pulse. Moreover, they discovered that the pulse inverts spins at any pulse amplitude exceeding a threshold. This insensitivity to RF variations makes adiabatic pulses appealing for both MR imaging and spectroscopy.
However, adiabatic inversion pulses have been unsuitable to selective spin echo generation because they leave a nonlinear phase variation across the slice. This phase cannot be refocused with linear gradients and causes signal loss when integrated in the projection through the slice. Kunz, Magnetic Resonance in Medicine (1987), showed that a "small-tip" .pi./2 pulse - essentially the adiabatic inversion pulse at exact amplitude to achieve a .pi./2 rotation could generate equal and opposite phase to compensate the .pi. pulse. A drawback of his technique is that both the .pi./2 rotation and its consequent phase compensation sensitive to RF variations.
Bendall et al., Magnetic Resonance in Medicine 1987 have recently proposed a selective refocusing pulse or spectroscopy and surface coil imaging applications. Their innovation was to reflect the second half of the excitation trajectory during an inversion pulse. For selective excitation using gradients, this reflection would require reversing the gradient amplitude during the pulse, and it would force the second half of the pulse to compensate for the phase accumulated during the first half of the pulse. Although this compensation is immune to RF amplitude variations, off-resonance spins are not completely refocused since one cannot switch the inhomogeneity field